Identification of new DNA markers close to the myotonic dystrophy locus

Autism spectrum conditions in myotonic dystrophy type 1: a study on 57 individuals with congenital and childhood forms

Myotonic dystrophy type 1 (DM1) is an autosomal dominant disorder, caused by an expansion of a CTG triplet repeat in the DMPK gene. The aims of the present study were to classify a cohort of children with DM1, to describe their neuropsychiatric problems and cognitive level, to estimate the size of the CTG expansion, and to correlate the molecular findings with the neuropsychiatric problems. Fifty-seven children and adolescents (26 females; 31 males) with DM1 (CTG repeats > 40) were included in the study. The following instruments were used: Autism Diagnostic Interview-Revised (ADI-R), 5-15, Griffiths Mental Development Scales, and the Wechsler Scales. Based on age at onset and presenting symptoms, the children were divided into four DM1 groups; severe congenital (n = 19), mild congenital (n = 18), childhood (n = 18), and classical DM1 (n = 2). Forty-nine percent had an autism spectrum disorder (ASD) and autistic disorder was the most common diagnosis present in 35% of the subjects. Eighty-six percent of the individuals with DM1 had mental retardation (MR), most of them moderate or severe MR. ASD was significantly correlated with the DM1 form; the more severe the form of DM1, the higher the frequency of ASD. The frequency of ASD increased with increasing CTG repeat expansions. ASD and/or other neuropsychiatric disorders such as attention deficit hyperactivity disorder, and Tourette's disorder were found in 54% of the total DM1 group. In conclusion, awareness of ASD comorbidity in DM1 is essential. Further studies are warranted to elucidate the molecular etiology causing neurodevelopmental symptoms such as ASD and MR in DM1.

Correlations between individual clinical manifestations and CTG repeat amplification in myotonic dystrophy

Myotonic dystrophy (DM) is a multisystemic disease caused by the expansion of a CTG repeat, located in the 3'-untranslated region of the DMPK gene. The number of CTG repeats broadly correlates with the overall severity of the disease. However, correlations between CTG repeat number and presence/absence or severity of individual clinical manifestations in the same patients are yet scarce. In this study the number of CTG repeats detected in blood cells of 24 DM subjects was correlated with the severity of single clinical manifestations. The presence/absence of muscular atrophy, respiratory insufficiency, cardiac abnormalities, diabetes, cataract, sleep disorders, sterility or hypogonadism is not related to the number of CTG repeats. Muscular atrophy and respiratory insufficiency are present with the highest frequency, occurring in 96 and 92% of the cases, respectively. A significant correlation was found with age of onset (r = -0.57, p<0.01), muscular disability (r = 0.46, p<0.05), intellective quotient (r = -0.58, p<0.01) and short-term memory (r= -0.59, p<0.01). Therefore, the CTG repeat number has a predictive value only in the case of some clinical manifestations, this suggesting that pathogenetic mechanisms of DM may differ depending on the tissue.

Homozygous myotonic dystrophy: clinical and molecular studies of three unrelated cases

We report the clinical and molecular study of three unrelated homozygous myotonic dystrophy patients. In the first family, the homozygous patient shows the classical form of the disease with two DM alleles of very different expansion sizes (1000 and 60 repeats). In the second family, the homozygous patient is mildly affected and carries a minimally expanded allele (64 repeats) and a "normal" allele (38 repeats) that increases in size when transmitted. Such an intergenerational expansion of an allele in this range of repeats has not been reported to date. The third homozygous case has late onset bilateral cataracts as the only symptom. She has two minimally expanded alleles (51 and 120 repeats) that showed different intergenerational enlargement during transmission to the next generation.

Myotonic dystrophy: molecular analysis of Israeli patients

Myotonic dystrophy (DM) is an autosomal dominant, multisystem disorder and the most common adult form of muscular dystrophy. The age of onset and degree of severity of DM is highly variable. The biochemical defect in DM is unknown. DM was the first autosomal disorder to be localised by genetic linkage to protein markers (Lu, Se, C3), and assigned to chromosome 19. Linkage studies in DM families using RFLPs as polymorphic markers refined the mapping position to 19q13.1-13.2 distal to the BCL3, apoCII, CKM and ERCCI genes. Based on the linkage data, healthy individuals from DM families request a pre-symptomatic test. The information is of use in planning their family, if at high risk they can choose to have prenatal diagnosis. We have studied ten unrelated DM families by linkage analysis. The DNA probes to detect the various RFLPS were either from the vicinity of BCL3, ApoCII, CKMM and ECCRI genes or anonymous DNA probes. Linkage analysis in the DM families enabled us to determine the carrier status of healthy individuals and to perform prenatal diagnosis at a confidence of > 99%. In two families the DM diagnosis was in doubt and we did not include them in the combined analysis. Linkage disequilibrium was noted with two RFLPs pDIO/Pst1 and p37.1/BamH1. Both DNA probes were isolated by Shaw and his group in Cardiff. In six out of eight families, the DM chromosome was associated with allele 3 of pDIO/Pst1 and allele 1 of p37.1/BamH1.(ABSTRACT TRUNCATED AT 250 WORDS)

Myotonic dystrophy: size- and sex-dependent dynamics of CTG meiotic instability, and somatic mosaicism

Myotonic dystrophy (DM) is a progressive neuromuscular disorder which results from elongations of an unstable (CTG)n repeat, located in the 3' untranslated region of the DM gene. A correlation has been demonstrated between the increase in the repeat number of this sequence and the severity of the disease. However, the clinical status of patients cannot be unambiguously ascertained solely on the basis of the number of CTG repeats. Moreover, the exclusive maternal inheritance of the congenital form remains unexplained. Our observation of differently sized repeats in various DM tissues from the same individual may explain why the size of the mutation observed in lymphocytes does not necessarily correlate with the severity and nature of symptoms. Through a molecular and genetic study of 142 families including 418 DM patients, we have investigated the dynamics of the CTG repeat meiotic instability. A positive correlation between the size of the repeat and the intergenerational enlargement was observed similarly through male and female meioses for < or = 0.5-kb CTG sequences. Beyond 0.5 kb, the intergenerational variation was more important through female meioses, whereas a tendency to compression was observed almost exclusively in male meioses, for > or = 1.5-kb fragments. This implies a size- and sex-dependent meiotic instability. Moreover, segregation analysis supports the hypothesis of a maternal as well as a familial predisposition for the occurrence of the congenital form. Finally, this analysis reveals a significant excess of transmitting grandfathers partially accounted for by increased fertility in affected males.

Characterization of a YAC and cosmid contig containing markers tightly linked to the myotonic dystrophy locus on chromosome 19

Myotonic dystrophy (DM) is caused by a defect in an unknown gene that maps to 19q13.3, flanked by the tightly linked markers ERCC1 on the proximal side and D19S51 on the distal side. We report the isolation and characterization of overlapping YAC and cosmid clones around D19S51 for the construction of a physical map around this locus. The resulting contig contains the markers D19S51 and D19S62 (another new marker tightly linked to the DM locus) and the distal breakpoint of a radiation hybrid cell line used in the physical mapping of the DM region. We have compared the restriction maps of the YACs and cosmids with that of the genome to investigate the fidelity of these clones.

Physical and genetic characterization of the distal segment of the myotonic dystrophy area on 19q

The mutation involved in myotonic dystrophy (DM) has been mapped to the region between the ERCC1 DNA repair gene and the anonymous D19S51 locus on 19q13.3. Starting at locus D19S112 (probe pX75b), which served as a novel entry site for this chromosome region, we have established a cosmid contig of approximately 200 kb. In the contig, a gene expressed in the brain and a highly informative, 12-allele (TG)n variable simple sequence motif (VSSM) were identified. With this marker, designated X75b-VSSM, a highly characteristic size distribution of alleles linked with DM, which differed significantly from that on normal chromosomes, was observed. Combining our physical mapping and genetic data, we show that the X75b-VSSM marker is the closest distal to DM, thus excluding the DM mutation from the entire telomeric portion of the ERCC1-D19S51 region.

Radiation-reduced hybrids for the myotonic dystrophy locus

The myotonic dystrophy (DM) gene maps to the long arm of human chromosome 19 and is flanked by markers ERCC1 and D19S51. Also mapping to this region is the polio virus receptor gene (PVS). To produce more markers for this interval, we have constructed radiation-reduced hybrids by selecting for the retention of ERCC1 and for the loss of PVS. One of the cell lines produced has been characterized extensively and contains about 2 Mb of human DNA derived exclusively from chromosome 19, and includes ERCC1 and D19S51. Phage libraries constructed from DNA of this cell line have been screened and several new markers identified, including two for which cDNAs have been isolated. These represent candidate genes for DM. The new markers have also been used to extend the long-range restriction map of this region.

Molecular basis of myotonic dystrophy: expansion of a trinucleotide (CTG) repeat at the 3' end of a transcript encoding a protein kinase family member

Using positional cloning strategies, we have identified a CTG triplet repeat that undergoes expansion in myotonic dystrophy patients. This sequence is highly variable in the normal population. PCR analysis of the interval containing this repeat indicates that unaffected individuals have been 5 and 27 copies. Myotonic dystrophy patients who are minimally affected have at least 50 repeats, while more severely affected patients have expansion of the repeat containing segment up to several kilobase pairs. The CTG repeat is transcribed and is located in the 3' untranslated region of an mRNA that is expressed in tissues affected by myotonic dystrophy. This mRNA encodes a polypeptide that is a member of the protein kinase family.

Detection of an unstable fragment of DNA specific to individuals with myotonic dystrophy

Myotonic dystrophy (DM) is the most common form of adult muscular dystrophy, with a prevalence of 2-14 per 100,000 individuals. The disease is characterized by progressive muscle weakness and sustained muscle contraction, often with a wide range of accompanying symptoms. The age at onset and severity of the disease show extreme variation, both within and between families. Despite its clinical variability, this dominant condition segregates as a single locus at chromosome 19q13.3 in every population studied. It is flanked by the tightly linked genetic markers ERCC1 proximally and D19S51 distally; these define the DM critical region. We report the isolation of an expressed sequence from this region which detects a DNA fragment that is larger in affected individuals than in normal siblings or unaffected controls. The size of this fragment varies between affected siblings, and increases in size through generations in parallel with increasing severity of the disease. We postulate that this unstable DNA sequence is the molecular feature that underlies DM.

Cloning of the essential myotonic dystrophy region and mapping of the putative defect

Myotonic dystrophy is a common dominant disorder (global incidence of 1:8,000) with variable onset and a protean nature of symptoms mainly involving progressive muscle wasting, myotonia and cataracts. To define the molecular defect, we have cloned the essential region of chromosome 19q13.3, including proximal and distal markers in a 700-kilobase contig formed by overlapping cosmids and yeast artificial chromosomes (YACs). The central part of the contig bridges an area of about 350 kilobases between two new flanking crossover borders. This segment has been extensively characterized through the isolation of five YAC clones and the subsequent subcloning in cosmids from which a detailed EcoRI, HindIII, MluI and NotI restriction map has been derived. Two genomic probes and two homologous complementary DNA probes were isolated using the cosmids. These probes are all situated within approximately 10 kilobases of genomic DNA and detect an unstable genomic segment in myotonic dystrophy patients. The length variation in this segment shows similarities to the instability seen at the fragile X locus. The physical map location and the genetic characteristics of the length polymorphism is compatible with a direct role in the pathogenesis of myotonic dystrophy.

Myotonic dystrophy: another case of too many repeats?

Myotonic dystrophy (DM) is an adult form of muscular dystrophy affecting about 1 in 8,000 individuals in most populations. Although common symptoms include progressive muscle weakness and stiffness, it is characterised by a heterogeneous clinical picture. Despite this variation in both the nature and severity of the symptoms seen in affected individuals, DM is genetically homogeneous, segregating as a single locus on the proximal long arm of human chromosome 19. As the biochemical abnormality underlying the disease was unknown, a reverse genetics (or positional cloning) strategy for identifying the gene responsible was adopted. The resulting collaborative effort culminated in the detection of the molecular mutation event and the gene within which it lies: the expansion of a trinucleotide repeat (CTG) at the 3' end of a gene encoding a member of the cyclic AMP-dependent protein kinase family. This has diagnostic implications since an easy, reliable and predictive test can now be offered to individuals with a family history of DM. These findings are also a prerequisite for further studies concerning the biochemical and physiological aetiology of DM and possible therapeutic strategies. In addition, the striking similarity between findings at the DNA level in DM and those in fragile X syndrome and spinal and bulbar muscular atrophy suggests that the mechanism leading to the increase in copy number of trinucleotide repeats at particular loci may be responsible for a number of other genetic diseases.

Detection of linkage disequilibrium between the myotonic dystrophy locus and a new polymorphic DNA marker

We have examined the linkage of two new polymorphic DNA markers (D19S62 and D19S63) and a previously unreported polymorphism with an existing DNA marker (ERCC1) to the myotonic dystrophy (DM) locus. In addition, we have used pulsed-field gel electrophoresis to obtain a fine-structure map of this region. The detection of linkage disequilibrium between DM and one of these markers (D19S63) is the first demonstration of this phenomenon in a heterogeneous DM population. The results suggest that at least 58% of DM patients in the British population, as well as those in a French-Canadian subpopulation, are descended from the same ancestral DM mutation. We discuss the implications of this finding in terms of strategies for cloning the DM gene, for a possible role in modification of risk for prenatal and presymptomatic testing, and we speculate on the origin and number of existing mutations which may result in a DM phenotype.